1. Field of the Invention
The present invention relates to an inkjet printing apparatus used in printers and composite machines.
2. Description of the Related Art
A wide range of research and development has been conducted on the inkjet printing technology because of its advantage of being able to manufacture printers and composite machines with a color print capability at a relatively low cost. The inkjet printing apparatus have been in wide use as commercially available devices of printers and composite machines. The inkjet printing apparatus generally use a print head formed with a plurality of ink ejection ports to eject inks of multiple colors. Such a print head has been known to cause the following troubles when a solvent in ink evaporates from the ink ejection ports increasing a viscosity of ink near the ink ejection ports. They include such phenomena as ejection direction deflections where the ink droplet ejection direction becomes deflected and ejection failures where ink droplets cannot be ejected.
Countermeasures against these troubles incorporated in general inkjet printing apparatus include a capping unit to minimize a vaporization of ink solvent from the ink ejection ports and a viscous ink expelling unit to expel useless viscous ink out of the ejection ports to the outside of a print medium before starting a printing session. Further, because foreign matters adhering to the print head surface near the ink ejection ports can cause ejection direction deflections and ejection failures, the print head also has a wiping unit to wipe clean the ink ejection port surface of the print head.
For reduced size and cost, many of the inkjet printing apparatus of recent years have a plurality of ink ejection ports for a plurality of color inks formed in essentially a single ink ejection port surface so that it can be wiped by a single wiper. Such an inkjet printing apparatus has been known to produce a so-called color mixing phenomenon in which some color inks get into ink ejection ports of other color inks during the wiping operation.
Japanese Patent Laid-Open No. H05-261942(1993) discloses a technique to deal with this problem. The inkjet printing method disclosed in the Japanese Patent Laid-Open No. H05-261942(1993) prevents the color mixing problem by executing a cleaning ejection following the wiping operation before starting the printing operation on a print medium.
However, experiments conducted by the inventors of this invention have found that, under the following circumstance, the color mixing phenomenon can still occur even if the cleaning ejection following the wiping operation has been done before the printing operation on a print medium is started. The experimental operations were conducted in the following procedure. First, following a wiping operation, a cleaning ejection is performed using a volume of ink for each of color inks that prevent the color mixing from occurring if a printing operation is done on a print medium immediately after the cleaning ejection. (This cleaning ejection performed following the wiping operation is also called a post-wiping cleaning ejection; and the volume of ink used in the post-wiping cleaning ejection is also called an optimal ink volume for post-wiping cleaning ejection.) Then, after a lapse of a predetermined time following the cleaning ejection, the printing operation is performed on a print medium. With this experiment it has been confirmed that the color mixing can result when the printing operation is done more than the predetermined time after the cleaning ejection.